Formula (5.2) itself is well-known (e.g., (3.1.34), p.141 in [1]})
for \(f\)  in some classes. We can refer also to
Gorenflo and Mainardi [2]},
Gorenflo, Mainardi and Srivastva [3]},
Gorenflo and Rutman [4]}, Luchko and Gorenflo [5]}.
